Known air separation modules (ASMs) are found in aircraft fuel tank flammability reduction systems. The ASM removes some oxygen from air to generate nitrogen-enriched air (NEA), which then flows into fuel tank ullage, a region in the tank that often contains evaporated fuel (such as, fuel vapor). The NEA may reduce flammability of the fuel tank ullage. Federal Aviation Administration (FAA) regulations require that new and in-service transport aircraft include systems for enhancing the safety of aircraft fuel tanks. Unfortunately, ASMs add weight to aircraft. Accordingly, decreased weight of air separation modules is desirable.